1. Field of the Invention The present invention relates to a photoconductive element generating and detecting electromagnetic waves by the radiation of light and a method of manufacturing the same, and in particular, to a photoconductive element generating and detecting electromagnetic waves including at least a part of a frequency range of 30 GHz to 30 THz (hereinafter referred to as “terahertz wave”) as frequency components by the radiation of light, a method of manufacturing the same, and a sensor device using the same.
2. Description of the Related Art
A non-destructive sensing technique using the terahertz wave has been developed in recent years. The electromagnetic wave in this frequency range finds its application in fields including an imaging technique for a safe fluoroscopic device substituting X rays, a spectroscopic technique in which absorption spectrum and complex dielectric constant inside a substance are determined to survey bonding state, a technique for analyzing biological molecules and a technique for evaluating carrier density and mobility.
A photoconductive element with an antenna serving also as an electrode provided on a photoconductive film with a thickness in the range of micrometers deposited on a substrate is suitably used as a terahertz wave generating unit (refer to Japanese Patent Application Laid-Open No. H10-104171. FIG. 7 illustrates an example of the configuration of the photoconductive element. A substrate 130 has, for example, a radiation-treated silicon-on-sapphire structure in which a silicon film as a photoconductive material is deposited on a sapphire substrate. In general, LT-GaAs grown on a GaAs substrate at low temperature is frequently used as a photoconductive film. A dipole antenna 138 formed on the surface includes a pair of dipole feeders 138a and 138b and a pair of dipole arms 139a and 139b. Optical pulses are focused in a gap 133. A voltage is applied across the gap to generate a terahertz wave pulse. If optical current is detected without the voltage applied across the gap, the terahertz wave pulse can be detected. A substrate lens 136 has a role to combine an electromagnetic wave from a slab mode (substrate mode) confined in the substrate 130 to a free-space radiation moment and to control the radiation angle of electromagnetic-wave propagation mode in space.
As shown in FIG. 7, passing through the substrate, a terahertz wave is generated, which is a typical configuration. Another configuration has been proposed in which a terahertz wave to be generated is taken out of the surface of a photoconductive film to prevent the wave from being dispersed and absorbed in the substrate (refer to Applied Physics Letters, vol. 85, p. 164, 2004).
The photoconductive film functioning to generate such a terahertz wave pulse is made only of a thin film formed on the substrate and the substrate has a role to hold the thin film. However, the substrate has the property of decreasing the transmittance of terahertz waves generated from the photoconductive film, decreasing efficiency in the generation of terahertz waves radiating from the other side of the substrate.
The proposal described in Applied Physics Letters, vol. 85, p. 164, 2004, which is of surface-generation type, has a configuration in which electromagnetic waves are reflected against incident laser beams, which complicates an optical locatement. In addition, the substrate has a confinement effect as a dielectric substance, for this reason, efficiency in the generation of electromagnetic waves on the surface cannot be practically increased so much.
A GaAs substrate is required when LT-GaAs is used as a photoconductive film. However, the substrate has a significant absorption due to optical phonon in the vicinity of a frequency of 7 THz. This lacks terahertz waves in the range of this absorption, causing a bottleneck in performing a terahertz-wave spectroscopy. The absorption is small in the surface-generation type disclosed in Applied Physics Letters, vol. 85, p. 164, 2004, however, it cannot be avoided.